1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to an apparatus and a method for driving a lamp of a liquid crystal display device that provide an improved range of lamp brightness.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device controls light transmittance of liquid crystal cells in accordance with data signals using a plurality of control switches, to thereby display an image. Further, an LCD device has broad applications in office automation equipment and audio/video equipment, because of its high image quality, lightness, thin thickness, compact size, and low power consumption.
An LCD device is a non-self-luminous display device and requires an external light source, such as a backlight device. There are two types of LCD backlight devices: a direct type and a light guide type. The direct type backlight device has a plurality of lamps arranged in a plane and a diffusion plate installed between the lamps and a liquid crystal display panel to fixedly maintain a distance between the lamps and the liquid crystal display panel. In contrast, the light guide type backlight device has a lamp installed at an outer area of a flat panel and a transparent light guide to direct light onto an entire-surface of the liquid crystal panel.
FIG. 1 is a schematic block diagram illustrating a liquid crystal display device according to the related art. In FIG. 1, an LCD device includes a liquid crystal display panel 20 having liquid crystal cells Clc arranged in a matrix-like manner at intersections between data lines DL and gate lines GL. In particular, the liquid crystal display panel 20 has liquid crystal formed between an upper substrate and a lower substrate and includes a spacer (not shown) for fixedly maintaining a distance between the upper substrate and the lower substrate. A color filter, a common electrode, and a black matrix (not shown) are formed on the upper substrate of the liquid crystal display panel 20, and a thin film transistor TFT is formed in each of the liquid crystal cells Clc on the lower substrate of the liquid crystal display panel 20.
In addition, an LCD driving apparatus includes a data driver 4 for applying data signals to the data lines DL, a gate driver 6 for applying gate signals to the gate lines GL, and a timing controller 8 for controlling the data driver 4 and the gate driver 6. For example, the thin film transistor TFT of each of the liquid crystal cells Clc applies a data signal from a respective one of the data lines DL to the liquid crystal cell Clc in response to a scanning signal from a respective one of the gate lines GL. Accordingly, the thin film transistor TFT is turned on when a scanning signal from the respective gate line GL, i.e., a gate high voltage, is supplied thereto, thereby supplying a pixel signal from the data line DL to the liquid crystal cell Clc. Further, the thin film transistor TFT is turned off when a gate low voltage from the respective gate line GL is supplied thereto, thereby maintaining the pixel signal charged in the liquid crystal cell Clc.
In FIG. 1, the liquid crystal cell Clc is expressed as a capacitor equivalent and also includes a pixel electrode (not shown) connected to the thin film transistor TFT and facing the common electrode with the liquid crystal therebetween. Further, each of the liquid crystal cells Clc includes a storage capacitor Cst for stably maintaining the charged pixel signal till the next pixel signal is charged. The storage capacitor Cst is formed between the previous gate line and the pixel electrode. As a result, in the liquid crystal cell Clc, the arrangement state of the liquid crystal having dielectric anisotropy is changed in accordance with the pixel signal charged through the thin film transistor TFT to control light transmissivity, such that the liquid crystal cell realizes gray.
The timing controller 8 may re-align a digital video data supplied from a digital video card (not shown) by red, green and blue. The video data re-aligned by the timing controller 8 is supplied to the data driver 4. Also, the timing controller 8 generates a data control signal and a gate control signal by use of a horizontal/vertical synchronization signal. The data control signals including a dot clock, a source shift clock, a source output enable, and a polarity inversion signal are supplied to the data driver 4. The gate signals including a gate start pulse, a gate shift clock, and a gate output enable are supplied to the gate driver 6.
In addition, the data driver 4 supplies the pixel signals of one line portion to the data lines DL every horizontal line in response to the data control signals from the timing controller 8. In particular, the data driver 4 converts the digital video data from the timing controller 8 into an analog video signal by use of a gamma voltage from a gamma voltage generator (not shown). The data driver 4 includes a plurality of data drive integrated circuit (hereinafter, referred to as “IC”) which are separately driving the data lines DL. Further, the gate driver 6 sequentially supplies the gate high voltage to the gate lines GL in response to the gate control signals from the timing controller 8, and supplies the gate low voltage in the remaining period when the gate high voltage is not supplied to the gate lines GL.
Furthermore, the LCD driving apparatus includes an inverter circuit 50 for driving a backlight unit 30. The inverter circuit 50 applies a driving voltage or a driving current for driving the backlight unit 30. The backlight unit 30 generates light corresponding to the driving voltage or the driving current from the inverter circuit 50 to irradiate light to the liquid crystal display panel 20.
FIG. 2 is a schematic block diagram of the inverter circuit shown in FIG. 1. As shown in FIG. 2, the backlight unit 30 includes a lamp 21 to generate light. The lamp 21 includes a glass tube, an inert gas within the glass tube, a high voltage electrode at one end of the glass tube, and a low voltage electrode at another end of the glass tube. The inert gas is charged in the glass tube, and phosphorus is spread over the inner wall of the glass tube. For example, if a high AC voltage 24 is applied from the inverter circuit 50 to the lamp 21, electrons are emitted from the low voltage electrode to collide with the inert gas inside the glass tube, thereby increasing the amount of electrons by geometrical progression. The increased electrons cause electric current to flow in the inside of the glass tube, thus the inert gas is excited to emit ultraviolet ray. The ultraviolet ray collides with the luminous phosphorus spread over the inner wall of the glass tube to then emit a visible ray.
The inverter circuit 50 includes an inverter IC 32, a transformer 34, a feedback circuit 36, and a pulse width modulation (PWM) circuit 38. The inverter IC 32 includes at least one switching device (not shown) to convert a supply voltage Vcc supplied from a voltage source (not shown) into an AC waveform. The AC waveform is supplied to the transformer 34 to form the high AC voltage 24, and the high AC voltage 24 then is supplied to the backlight unit 30 (shown in FIG. 1) to drive the lamp 21. In particular, the AC waveform is induced by the winding ratio of the primary winding and the secondary winding of the transformer 34, and the high AC voltage waveform 24 induced by the secondary winding of the transformer 34 is supplied to the high voltage electrode of the lamp 21.
In addition, the feedback circuit 36 detects a tube current of the lamp 21 and outputs a feedback signal FB to the PWM circuit 38. The feedback circuit 36 includes a resistor, a diode and the like, and generates the feedback signal FB to correspond to the tube current. Further, the PWM circuit 38 generates a switching control signal SCS to control the switching device of the inverter IC 32 based on the feedback signal FB.
FIG. 3 is a waveform diagram illustrating an AC voltage waveform for driving the lamp shown in FIG. 2 in a continuous mode. As shown in FIG. 3, in a continuous mode, the AC voltage waveform 24 continuously oscillates between the positive and negative peak voltages. As a result, when a continuous mode driving method is employed, the lamp 21 (shown in FIG. 2) is on continuously.
FIG. 4 is a waveform diagram illustrating an AC voltage waveform for driving the lamp shown in FIG. 2 in a burst mode. As shown in FIG. 4, in a burst mode, the AC voltage waveform 24 oscillates between the positive and negative peak voltages only during a first designated period Ton and remains at zero during a second designed period Toff within a time period T. As a result, when a burst mode driving method is employed, the lamp 21 (shown in FIG. 2) is on only during the first designated period Ton.
FIG. 5 is a graph illustrating brightness of the lamp when the AC voltage waveforms shown in FIGS. 3 and 4 are applied thereto. In FIG. 5, a solid line A represents brightness of the lamp 21 (shown in FIG. 2) when the AC voltage waveform of the continuous mode driving method shown in FIG. 3 is applied thereto. For example, the continuous mode driving method provides a brightness range of 300 nit to 390 nit corresponding to the tube current being between 5.0 mA and 8.0 mA. Further, in FIG. 5, a dotted line B represents brightness of the lamp 21 (shown in FIG. 2) when the AC voltage waveform of the burst mode driving method shown in FIG. 4 is applied thereto. For example, the burst mode driving method provides a brightness range of 140 nit to 390 nit corresponding to the tube current being between 4.0 mA to 8.0 mA.
Thus, although the lamp driven by the continuous mode driving method provides higher brightness, the continuous mode driving method has a disadvantage in that the power consumption of the inverter circuit 32 (shown in FIG. 2) and the backlight unit 30 (shown in FIG. 1) is high because the high AC voltage waveform is continuously supplied to the lamp 21. However, the burst mode driving method provides a limited brightness range. Moreover, the liquid crystal display device according to the related art has another disadvantage in that brightness within the dot hatched area C cannot be realized.